This invention relates to a device for drying, in particular continuously, particulate material according to the preamble of claim 1 and a method for providing, in particular continuously, dried particulate material according to claim 13.
Plastics (i.e., synthetic resin materials) are often supplied in the form of granules or pellets for further processing. However, in order to process such plastic granules, a certain moisture content must not be exceeded. Therefore, plastic granules are dried. In the case of non-hygroscopic plastics, the moisture adhering to the surface of the plastics is removed by drying. In granulation of hygroscopic plastics, the moisture must also be removed from the pores in the interior of the granules.
Dryer systems and equipment for bulk particulate materials usually comprise an essentially cylindrical and upright container and/or drying hopper, which has a feed port for particulate material to be dried at the top end and has a closable discharge opening for particulate material at the bottom end which tapers in a conical shape, for example. Near the discharge opening, an inlet pipe for dry air leads from a dryer to the interior of the cylindrical container and an exhaust air pipe is provided near the feed port. During operation, particulate material introduced into the container, e.g., a powder or plastic granules or plastic pellets, falls through the container against a countercurrent of hot dry air which picks up the moisture entrained by the particulate material and removes it as exhaust air from the container.
The discharge of (dry) particulate material depends on the demand for dry particulate material for bulk processing machines connected to the discharge opening of the dryer, e.g., injection molding machines or extruders in the case of plastics. First, adequately dried granules must always be kept available for this purpose in the dryer. Second, the granules dwelling too long at the target temperature or overdrying of the granules must be prevented.
In particular, drying of granules of hygroscopic plastics requires a great input of energy to overcome the binding of moisture in the pores and to evaporate the liquid so that it can then be removed via the gas phase. This leads first to the problem of having to introduce a sufficient quantity of heat into the granules to be dried but second to the problem of being able to retain a required residual moisture content in the product after drying. The term “overdrying” refers to the effect whereby drying has been continued until the moisture content of the product is below the level required for the residual moisture content. In addition, the term “overdrying” also encompasses the effect whereby destructive changes in the plastic may occur when the residence time at high temperatures is too long. These may be chemical changes, for example, which may be caused in particular by oxidation, when drying is performed with air as the drying medium. Changes in the viscosity of the plastic are also of great importance for the processability in the downstream equipment.
When drying plastics, e.g., PET, a relatively low residual moisture content adhering to the granules or the pellets and/or bound in the particulate material or the granules must be removed. This requires not only dry air heating of the granules that are to be heated to a preselected target temperature of 170° C., for example, depending on the type of plastic, but also the granules must be kept at this temperature for a specified residence time.
It is known from German Patent No. DE 3,131,471 that with regard to the energy balance of the dryer system, the quantity of air throughput through the particulate material drying hopper should be reduced when there is an increase in the exhaust air temperature and should be increased when there is a decline in the exhaust air temperature. This should achieve the result that no more than the heat required for drying is supplied by the drying air to a dryer system that is operated at only partial load. Nevertheless, the energy balance remains unfavorable because practically all the particulate material present in the container must be heated before the exhaust air temperature has risen to the extent that the air throughput is reduced accordingly. A direct coordination with a specifiable residence time of the particulate material within a target temperature range is not possible on the basis of German Patent No. DE 3,131,471.
European Patent No. EP 487,829 discloses a particulate material dryer system in which instead of the exhaust air temperature, the temperature of the particulate material is detected by a temperature measuring device mounted in the container for controlling the air flow rate through the drying hopper. If the particulate material temperature detected by the temperature measuring device is below a specific limit, a discharge of particulate material through the discharge opening of the container can be interrupted by a warning signal generated by the temperature measuring device via a signal converter. If the temperature of the particulate material temperature rises too much, the signal converter may generate a control signal which leads to throttling of the quantity flow of air supplied.
However, one disadvantage of this arrangement is that the inferences from a single measured temperature value as to the drying status of the entire particulate material are unreliable. Since the drying status cannot be detected through the entire particulate material, it is impossible to be sure that the particulate material to be discharged has the required residual moisture content and has not been overdried.
European Patent No. EP 711,641 describes a particulate material drying hopper in which a tree of several temperature sensors arranged a distance apart, one above the other, is provided, their outputs being connected to a signal processing unit which includes a programmable microprocessor. The microprocessor processes the temperature data from the temperature sensors as well as information that is entered regarding the type of granules to be dried, the respective target temperature and residence time as well as the desired granule throughput and the density of the granular material and determines from this the temperature sensors below which the respective granules must be heated to the target temperature, must remain for a sufficient period of time at this temperature and supplies the desired granule throughput for discharge.
The microprocessor releases the granules for discharge only when the temperature sensor thus determined signals the target temperature and thus the residence time has elapsed. One disadvantage is that the particulate material dryer device blocks the discharge at a lower granule demand than at the set granule throughput when, for example, one or more plastic processing machines connected to the particulate material dryer device have been shut down, and it continues blocking the discharge until the full throughput of granules has been provided.
It is also provided [in that invention] that intermittent drying of a bulk feedstream in the container is to be performed in such a way that the quantity to be discharged is adequately dry. Another disadvantage is that it is impossible to run a continuous operation because superimposed drying and discharge of the particulate material are not intended in that patent.
Despite the efforts of the prior art, there remains a need for improved control of particulate material drying systems.